Blackburn have already rejected one bid from the Saints, and Mowbray insists he is currently preparing for the new season with Armstrong in his plans.
The Rovers boss told Lancashire Telegraph : "I think we have to do that because there's no guarantee of a percentage of what we get for Adam coming back into being reinvested into the team.
"We have to wait and see. If Adam gets sold then I will ask the owners what we can take back from that deal."
Mowbray would have 'no issues' with sanctioning the sale of Armstrong, but has stressed money would need to be made available to reinvest into the squad.
He added: "My frustration is that I've talked about building a club, but you can't build a club if you're only signing loans. You have to sign players and polish them up.
"I've got no issues with selling Adam Armstrong because we buy him for a number and then sell him for multiples of that number, we reinvest that money and buy some more young players and make them better and then keep building your club.
"That process isn't finished because we don't know what will happen if we sell Adam, we'll ask the owners what money would be available."
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